Fix early arch detection & the 'Cannot find ELF base!' warning

Before this commit, when we did `gdb /bin/ls` and then `entry`, we could see a "Cannot find ELF base!" warning. This occured, because the `pwndbg.arch.ptrmask` was incorrectly set and then the `find_elf_magic` was using the `pwndbg.arch.ptrmask` which was 0xffFFffFF and made an early return. As a result of this early return, the "Cannot find ELF base!" warning was emitted.

The reason for incorrect early arch detection is that we used the `gdb.newest_frame().architecture().name()` API while the binary was not using in here:
```python
 @pwndbg.events.start
 @pwndbg.events.stop
 @pwndbg.events.new_objfile
 def update():
     m = sys.modules[__name__]

     try:
         m.current = fix_arch(gdb.newest_frame().architecture().name())
     except Exception:
         return

     m.ptrsize = pwndbg.typeinfo.ptrsize
     m.ptrmask = (1 << 8*pwndbg.typeinfo.ptrsize)-1
```

And if the binary was not running, we just returned early and did not set `pwndbg.arch.ptrsize` and `pwndbg.arch.ptrmask` at all, leaving them at their default values.

Now, those values were often eventually fixed, but this occured by chance as the `new_objfile` was executed!

Anyway, starting from this commit, we will detect the arch from `gdb.newest_frame().architecture().name()` only if the process is running and if it is not, we will fallback to the `show architecture` GDB command and parse it, hoping we detect the arch properly. In case we don't, or, we don't support the given arch, we raise a `RuntimeError` currently.

Also, as a side note: the `find_elf_magic` from `elf.py` can be optimized to instead of doing 4kB steps over pages, to just look at the begining of a page. Otherwise, if this doesn't work, we are most likely on a target that may not have an ELF magic/header at all, so we shouldn't bother about it. This fix is done in the next commit.

pwndbg/arch.py

@@ -20,8 +20,17 @@ fmt     = '=I'
 native_endian = str(sys.byteorder)
 
 
-def fix_arch(arch):
-    for match in ['x86-64', 'i386', 'mips', 'powerpc', 'sparc', 'aarch64']:
+def _get_arch():
+    not_exactly_arch = False
+
+    if pwndbg.proc.alive:
+        arch = gdb.newest_frame().architecture().name()
+    else:
+        arch = gdb.execute("show architecture", to_string=True).strip()
+        not_exactly_arch = True
+
+    # Below, we fix the fetched architecture
+    for match in ('x86-64', 'i386', 'aarch64', 'mips', 'powerpc', 'sparc'):
         if match in arch:
             return match
 
@@ -29,24 +38,20 @@ def fix_arch(arch):
     if 'arm' in arch:
         return 'armcm' if '-m' in arch else 'arm'
 
+    if not_exactly_arch:
+        raise RuntimeError("Could not deduce architecture from: %s" % arch)
+
     return arch
 
+
+
 @pwndbg.events.start
 @pwndbg.events.stop
 @pwndbg.events.new_objfile
 def update():
     m = sys.modules[__name__]
 
-    # GDB 7.7 (Ubuntu Trusty) does not like selected_frame() when EBP/RBP
-    # is not mapped / pounts to an invalid address.
-    #
-    # As a work-around for Trusty users, handle the exception and bail.
-    # This may lead to inaccurate results, but there's not much to be done.
-    try:
-        m.current = fix_arch(gdb.newest_frame().architecture().name())
-    except Exception:
-        return
-
+    m.current = _get_arch()
     m.ptrsize = pwndbg.typeinfo.ptrsize
     m.ptrmask = (1 << 8*pwndbg.typeinfo.ptrsize)-1